为什么在扩展TestCase类时无法找到或加载主类

Question

I encounter a interesting error by accident and I need you help me figure it out.

as for the below code every thing is ok:

public class HelloWorld{
    public static void main(String[] args){
        System.out.println("hello world");
    }
}

then as for the follow code, something interesting come.

import junit.framework.TestCase;

public class HelloWorld extends TestCase{
    public static void main(String[] args){
        System.out.println("hello world");
    }
}

when I run it, it says

cant find or load main class.

I know when jvm load HelloWorld class it will load TestCase class first. So I make the below code segment.

public class HelloWorld {
    public static void main(String[] args) throws ClassNotFoundException{
        Class.forName("junit.framework.TestCase");
    }
}

Exception in thread "main" java.lang.ClassNotFoundException: junit.framework.TestCase

I dont know why? Maybe the TestCase class is not on my classpath ? But as for the second code segment I ensure TestCase is on my classpath .

---

So my question is:

1. Why can't I load TestCase class?
2. Why can't it find or load main class when extend TestCase class?

Note: I know the print helloworld function has nothing with TestCase class which used to junit test but I want to figure out the reason that bring this suprise.

Answer 1

I dont know why? Maybe the TestCase class is not on my classpath ? But as for the second code segment I ensure TestCase is on my classpath.

 Class.forName("junit.framework.TestCase"); 

returns the Class object associated with TestCase class. It doesn t set the classpath.

That's why you rise the exception :

Exception in thread "main" java.lang.ClassNotFoundException: junit.framework.TestCase

You try to load a class which is not in the classpath.

To solve your problem with Eclipse, go in the properties windows of your project, then at the left, you have the Java Build Path option, go inside and look at the libraries tab. Here, you should add the JUnit library if you want JUnit to be in the classpath.